1. Field of the Invention
This invention relates generally to a communications time-delay circuits and more specifically to a continuously variable true time-delay (i.e., phase shift) circuits for radio frequency, microwave and millimeter wave signals using fiber optics.
2. Description of the Related Art
Some existing time-delay selective circuits are implemented using microwave or integrated optic switches to switch in or out an extra electrical path length--a length of cable for electrical transmission and a length of optical fiber in the case of fiber optic transmission. (See, U.S. Pat. No. 5,051,754, Newberg.) In order to continuously vary the time delay of the signal, the spacing between the source and the receiver, or the signal velocity, must change continuously. This is accomplished using either a direct path translator or a reflected path translator. However, the usefulness of any type of mechanical implementation will be severely limited by the precision and mechanical tolerance required. Other drawbacks of these techniques include the discrete nature of the incremented delay, complexity, size, weight and power requirements.
A popular method currently used for generating continuous phase shifts is vector modulation. This method provides a phase shift that is independent of frequency. However, this method suffers from problems of phase shift accuracy, limited bandwidth and high harmonic levels. A previous type of continuous true time-delay modulator is the piezo-electric optical fiber stretcher (See, U.S. Pat. No. 4,814,774, Herczfeld); however, this approach is limited by speed and size as well as by power supply requirements. Several other phase shifting methods include movable shortcircuit pistons (reflection mode), variactors, reciprocal and nonreciprocal ferrite devices and semiconductor diode designs. However, all of these methods are not only intended to provide a phase shift independent of frequency and suffer from limited bandwidth, amplitude modulation, signal distortion, etc.
Recent work by R. A. Soref (U.S. Pat. No. 4,671,604) teaches a system for wavelength dependent, tunable time delay system for electrical signals having a RF/electrical-to-optical converter with a selectable optical wavelength, a single-mode dispersive fiber, and an optical-to-RF/electrical converter. By selectively varying the wavelength of the optical signal, the electrical signal can be time delayed as desired in response to an electrical signal.